This invention relates to a magnetic sensor and, more particularly, to a magnetoresistive-type magnetic sensor which is readily adapted to sense the proximity of a magnetically permeable member.
Magnetic sensors are useful in instrumentation, magnetic readers, position detectors, and various other applications. Typically, the magnetic sensor is used as a so-called "contactless" switch wherein an output signal is generated, analogous to the closing or opening of a switch, when a particular magnetic event is sensed. Such a contactless switch often is used in servo control systems wherein a process or machine is controlled as a function of the output of the contactless switch.
A semiconductive magnetic reluctance element, such as a Hall effect device, has been proposed for use as a magnetic sensor. The Hall effect device, being a semiconductive transducer, exhibits undesirable temperature characteristics. Accordingly, when a Hall effect device is used, a temperature compensating circuit generally must be employed. Furthermore, correction and compensating circuitry also is provided, thus increasing the complexity and cost of a magnetic sensor using a Hall effect device.
Another magnetic sensor that has been proposed relies upon the ferromagnetic reluctance effect of a ferromagnetic metal. In one application of this ferromagnetic reluctance effect, the resistance of the ferromagnetic material changes in response to a relatively large external magnetic field in accordance with Mott's theory. In general, as the external magnetic field increases, the resistance of the ferromagnetic material decreases. This negative relationship between the magnetic field and the resistance of the ferromagnetic material typically is linear. An isotropic relationship with respect to the direction of the magnetic field obtains when the ferromagnetic material is heated to its Curie temperature. At lower temperatures, however, this isotropic relationship is minimal. Since the negative magnetic reluctance effect is useful only in the environment of relatively high magnetic fields, magnetic sensors which rely upon this effect exhibit limited utility in specialized applications.
In the presence of relatively small magnetic fields, some ferromagnetic materials exhibit a resistivity that varies anisotropically with the direction of the applied magnetic field. Magnetic sensors employing this ferromagnetic material have been formed of an insulating substrate with a thin film of ferromagnetic material deposited thereon to form ferromagnetic strips in zig-zag or serpentine configuration. Such ferromagnetic strips exhibit magnetoresistance, whereby the resistance of the strips varies anisotropically. The use of such magnetoresistive elements to detect a magnetic field is disclosed in U.S. Pat. Nos. 3,928,836, 4,021,728, 4,053,829 and 4,079,360, as well as in application Ser. Nos. 23,270, filed Mar. 23, 1979, and Ser. No. 237,115, filed Feb. 23, 1981, all assigned to the assignee of the present invention.
In the foregoing patents and patent applications, the magnetic sensor generally is comprised of two series-connected magnetoresistive elements having respective main current conducting paths which, typically, are perpendicular to each other. If a saturating bias magnetic field is supplied to both magnetoresistive elements, a predetermined output signal is produced. If the direction of the saturating magnetic field changes, the output signal will change as a function of the angle formed between the direction of the magnetic field and the main current conducting paths of the magnetoresistive elements.
In the magnetic sensors of the aforementioned type, wherein magnetoresistive elements are used, a maximum resistivity is exhibited by the magnetoresistive element when the direction of the applied magnetic field is parallel to the main current conducting path thereof. This resistivity is, however, a minimum when the applied magnetic field is perpendicular to the current conducting path. This anisotropic relationship is expressed in the Voight-Thomson equation: EQU R(.theta.)=R.sub.195 .multidot.sin.sup.2 .theta.+R.sub..parallel..multidot. cos.sup.2 .theta. (1)
In equation (1) above, .theta. represents the angle of the magnetic field relative to the current conducting path. That is, the angle of the magnetic field relative to the longitudinal direction of the magnetoresistive strip which is included in the magnetoresistive element. Also, in equation (1) above, R.sub..perp. represents the resistance of the magnetoresistive element when the magnetic field is applied in a direction perpendicular to the direction of current flowing therethrough; and R.sub..parallel. represents the resistance of the magnetoresistive element when the magnetic field is parallel to the direction in which the current flows therethrough.
Some examples of ferromagnetic metals which exhibit desirable magnetoresistive characteristics and which can be used in the aforedescribed magnetic sensors are nickel-cobalt (NiCo) alloy, nickel-iron (NiFe) alloy, nickel-aluminum (NiAl) alloy, nickel-manganese (NiMn) alloy and nickel-zinc (NiZn) alloy.
In magnetic sensors of the aforementioned type, a saturating bias field is applied to two coplanar series-connected magnetoresistive elements, and an external, movable flux source, such as a magnet, is moved with respect to the magnetic sensor. The flux, or external field, generated by the magnet combines vectorially with the bias field such that the resultant field sensed by the magnetic sensor exhibits a particular angle; and this angle is detected by its action upon the magnetoresistance of the elements (as set out in equation (1) above). As a result, the magnetic sensor produces an output signal which is a function of that angle and, thus, a function of the relative location of the external magnet. In U.S. Pat. No. 4,021,728, the direction of the bias field relative to the magnetoresistive elements is disturbed by the influence thereon of a movable, highly permeable member.
Unfortunately, in many magnetic sensors formed of magnetoresistive elements, the output signal produced as a function of the angle of the magnetic field therethrough varies significantly with changes in temperature. Furthermore, in such magnetic sensors, the bias field generally is supplied at an angle which is less than optimum. That is, relatively large changes in the angle of the resultant field through the magnetoresistive elements causes only correspondingly small changes in the output signal produced thereby. That is, the rate of change of the output signal with respect to the angle of the resultant field is relatively low. Hence, such magnetic sensors do not exhibit relatively high sensitivity and low temperature-dependency, as are desired.